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MCP3008T Datasheet(PDF) 21 Page - Microchip Technology

Part No. MCP3008T
Description  2.7V 4-Channel/8-Channel 10-Bit A/D Converters with SPI Serial Interface
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Manufacturer  MICROCHIP [Microchip Technology]
Direct Link  http://www.microchip.com
Logo MICROCHIP - Microchip Technology

MCP3008T Datasheet(HTML) 21 Page - Microchip Technology

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© 2008 Microchip Technology Inc.
DS21295D-page 21
MCP3004/3008
6.0
APPLICATIONS INFORMATION
6.1
Using the MCP3004/3008 with
Microcontroller (MCU) SPI Ports
With most microcontroller SPI ports, it is required to
send groups of eight bits. It is also required that the
microcontroller SPI port be configured to clock out data
on the falling edge of clock and latch data in on the
rising edge. Because communication with the
MCP3004/3008 devices may not need multiples of
eight clocks, it will be necessary to provide more clocks
than are required. This is usually done by sending
‘leading zeros’ before the start bit. As an example,
Figure 6-1 and Figure 6-2 shows how the MCP3004/
3008 can be interfaced to a MCU with a hardware SPI
port. Figure 6-1 depicts the operation shown in SPI
Mode 0,0, which requires that the SCLK from the MCU
idles in the ‘low’ state, while Figure 6-2 shows the sim-
ilar case of SPI Mode 1,1, where the clock idles in the
‘high’ state.
As is shown in Figure 6-1, the first byte transmitted to
the A/D converter contains seven leading zeros before
the start bit. Arranging the leading zeros this way
induces the 10 data bits to fall in positions easily
manipulated by the MCU. The MSB is clocked out of
the A/D converter on the falling edge of clock number
14. Once the second eight clocks have been sent to the
device, the MCU receive buffer will contain five
unknown bits (the output is at high-impedance for the
first two clocks), the null bit and the highest order 2 bits
of the conversion. Once the third byte has been sent to
the device, the receive register will contain the lowest
order eight bits of the conversion results. Employing
this method ensures simpler manipulation of the
converted data.
Figure 6-2 shows the same thing in SPI Mode 1,1,
which requires that the clock idles in the high state. As
with mode 0,0, the A/D converter outputs data on the
falling edge of the clock and the MCU latches data from
the A/D converter in on the rising edge of the clock.
FIGURE 6-1:
SPI Communication with the MCP3004/3008 using 8-bit segments
(Mode 0,0: SCLK idles low).
12
3
4
5
6
7
8
9
10 11 12 13 14 15 16
CS
SCLK
DIN
X = “Don’t Care” Bits
17 18 19 20 21 22
23 24
DOUT
NULL
BIT B9 B8
B7
B6 B5 B4 B3 B2 B1 B0
HI-Z
MCU latches data from A/D
Data is clocked out of
A/D converter on falling edges
converter on rising edges of SCLK
Don’t Care
SGL/
DIFF
D1
D2
Start
00
0
0
0
1
XX
X
X
DO
X
X
XX
X
X
XX
B7 B6 B5 B4 B3 B2 B1 B0
B9 B8
0
??
?
?
?
?
?
??
?
D1
D2
SGL/
DIFF
Start
Bit
(Null)
MCU Transmitted Data
(Aligned with falling
edge of clock)
Data stored into MCU receive
register after transmission of first
8 bits
Data stored into MCU receive
register after transmission of
second 8 bits
Data stored into MCU receive
register after transmission of last
8 bits
DO
?
?
00
?
MCU Received Data
(Aligned with rising
edge of clock)


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